1. Field of the Invention
The present invention relates to a light scanning unit for irradiating light onto a photoconductor, and an electrophotographic image forming apparatus including the light scanning unit. More particularly, the present invention relates to a light scanning unit with a long focal length from an image lens to a photoconductor, and an electrophotographic image forming apparatus including the light scanning unit.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a light scanning unit is used to irradiate light corresponding to image data onto a photoconductor for forming an electrostatic latent image on the photoconductor. A developing unit develops the electrostatic latent image into a visible image. The developing unit contains toner having a predetermined color.
In an image forming apparatus forming an image using a single color, a single light scanning unit is used, and in a color image forming apparatus four light scanning units are usually used for forming electrostatic latent images corresponding to black (K), yellow (Y), magenta (M), and cyan (C) image data. Alternatively, some color image forming apparatuses use a single light scanning unit that can emit four different light beams corresponding to black (K), yellow (Y), magenta (M), and cyan (C) image data sequentially, or two light scanning units each emitting two different light beams corresponding to two colors of image data sequentially.
Generally, in a light scanning unit, a focal length is defined from an imaging lens to a photoconductor. The developing unit of an image forming apparatus is replaced with a new one when the toner contained in the developing unit is consumed. To reduce the replacing frequency of the developing unit, the amount of toner contained in the developing unit should be increased and thus the size of the developing unit should be enlarged. To increase the size of the developing unit, the focal length of the light scanning unit should be long. Meanwhile, the resolution of a light scanning unit can be expressed by the following equation: d=k×λ×L/D, where d is the resolution, k is a proportional constant ranging from 1.2 to 2.0, λ is the wavelength of light, L is a focal length, and D is the effective thickness defined in a sub-scanning direction for an optical component, such as an imaging lens. To realize a long-focal-length light scanning unit, the focal length L should be large. In this case, the effective thickness D increases in proportion to the focal length L. Although optical components having a larger effective thickness should be manufactured more precisely, state of the art optical components, such as imaging lenses, are manufactured by plastic injection molding, and thus it is very difficult to manufacture precise optical components having a large effective thickness. Furthermore, optical components having a large effective thickness are sensitive to environmental conditions, such as temperature, thereby increasing the possibility of image quality deterioration.
Accordingly, a need exists for an electrophotographic image forming apparatus having an improved light scanning unit that has a long focal length.